The general trend in space observation is to increase the diameter of mirrors, both for future scientific missions for observing the universe and for observing Earth from for example a geostationary orbit. Thus, in the near future, there will be a need for extremely stable materials which allow a high degree of lightening to be achieved, while still being rigid and strong, and which will enable mirrors to be produced with a diameter greater than 2 m and with a mass per unit area of less than 25 kg/m2. To obtain dimensionally stable mirrors, materials having a very low CTE (coefficient of thermal expansion) are sought.
Optical structures, such as telescope structures, are also subject to very strict requirements in terms of dimensional stability so as to be able to guarantee the image quality and notably to preserve the alignments made on the ground between the optical components during firing and throughout the mission. In addition, their increasing dimensions require materials permitting a high level of lightening to be achieved while still being rigid and strong.
More particularly, the aim is to obtain materials having a coefficient of thermal expansion and mechanical properties suitable for optical applications in the space field. The expression “coefficient of thermal expansion suitable for space applications” is understood to mean a coefficient of thermal expansion of less than 1.3×10−6 K−1 around the ambient temperature and/or at low temperatures (T<150 K). The expression “material having mechanical properties suitable for aerospace applications” is understood to mean a material having a high Young's modulus, i.e. greater than 100 GPa, and a high measured flexural strength, i.e. greater than 100 MPa.
Silicon nitride (Si3N4) is a very good candidate for these applications as it has good mechanical properties. Notably, it has a high Young's modulus, equal to about 320 GPa, and a high measured flexural strength, i.e. greater than 700 MPa. However, silicon nitride has a non-zero, slightly positive, coefficient of thermal expansion.
β-eucryptite is a lithium aluminosilicate widely referred to by the acronym LAS, the composition of which is the following: (Li2O)x(Al2O3)y(SiO2)z, in which x, y and z are the respective molar fractions of lithium oxide Li2O, alumina Al2O3 and silica SiO2. The respective molar fractions of β-eucryptite are the following: x=1, y=1 and z=2.
β-eucryptite has the particular feature of having a highly negative coefficient of thermal expansion, that is to say it contracts when the temperature is raised. The coefficient of thermal expansion of a nanoscale or micron-size polycrystal of β-eucryptite is around −8×10−6 K−1 (K corresponding to degrees Kelvin). When the β-eucryptite is incorporated into a silicon nitride matrix it has a tendency to lower the coefficient of expansion of the composite thus produced.
A process for manufacturing a sintered ceramic composite that includes a step of blending β-eucryptite and silicon nitride powders in an aqueous or alcoholic solution is known. The blend is then heated to a temperature for sintering the silicon nitride.
The Applicant has found, without disclosing this, that it is not possible to obtain, from this known process, a composite having dimensional stability and mechanical properties suitable for optical applications in the space field. Specifically, it is possible to obtain, from the known process, a material having a coefficient of expansion suitable for space applications from a silicon nitride/6-eucryptite blend only if the blend has a β-eucryptite mass proportion at least equal to 60%. Now, β-eucryptite, which has a Young's modulus of around 70 GPa, lowers the mechanical properties of the composite in relation to those of the silicon nitride. The composite obtained has mechanical properties incompatible with optical applications in the space field.